A circularly-polarized-light-extracting optical element that is made to reflect either right- or left-handed circularly polarized component having a wavelength equal to the helical pitch of liquid crystalline molecules in a cholesteric liquid crystal layer and to transmit the other circularly polarized component has conventionally been known as an optical element using a cholesteric liquid crystal or the like.
To extract (selectively reflect) circularly polarized component in a broadened wave range, there has been proposed a circularly-polarized-light-extracting optical element composed of a laminate of a plurality of cholesteric liquid crystal layers having different pitches of liquid crystalline molecular helixes (Japanese Laid-Open Patent Publications No. 271731/1996 and No. 264907/1999). Further, Japanese Laid-Open Patent Publication No. 304770/1997, for example, discloses a polarized light source device and a liquid crystal display each containing such a circularly-polarized-light-extracting optical element.
The circularly-polarized-light-extracting optical element as described above is often used as a display member. As shown in FIG. 21, the circularly-polarized-light-extracting optical element 203 is often placed between circular polarizers (or elliptical polarizers) 201 and 202 arranged in the cross nicol disposition (combination of a right-handed circular polarizer and a left-handed circular polarizer), for example.
In the case where the circularly-polarized-light-extracting optical element is used as a display member in the above-described manner, it is necessary that the condition of polarization be uniform throughout the light-emitting surface of the display. It has been, however, found that bright and dark stripes can appear on the screen of a display to drastically lower the displaying quality of the display.
Through experiments and computer-aided simulations, we have made earnest studies to clear up the cause of the above-described phenomenon; and, as a result, we have found that this phenomenon occurs depending partly on the direction (director) in which liquid crystalline molecules are oriented on the surface of a circularly-polarized-light-extracting optical element.
In connection with the above phenomenon, we have also found the following: a circularly-polarized-light-extracting optical element made by laminating a plurality of cholesteric liquid crystal layers shows lowered optical activity depending partly on the constitution of the laminate of the two or more cholesteric liquid crystal layers; and, in particular, in the case where a cholesteric liquid crystal layer is directly applied to another cholesteric liquid crystal layer, the directors in planes in the vicinity of the interface of the two neighboring cholesteric liquid crystal layers are significant.
In a conventional circularly-polarized-light-extracting optical element as disclosed in Japanese Laid-Open Patent Publication No. 264907/1999, a plurality of cholesteric liquid crystal layers that have been made into films in advance are usually adhered either with an adhesive or thermally without using any adhesive.
In the case where an adhesive is used, the adhesion between the liquid crystalline molecules in the liquid crystal layer that has been made into a film and the molecules of the adhesive is required to be good. Therefore, not only the types of adhesives that can be used are limited, but also the resulting circularly-polarized-light-extracting optical element inevitably has a thickness increased by the thickness of the adhesive layer. In addition, the use of an adhesive brings such problems that reflection occurs at the interface of the adhesive layer and the liquid crystal layer due to the difference in refractive index between the two layers and that light extracted by the circularly-polarized-light-extracting optical element has the color of the adhesive layer itself.
In the case where liquid crystal layers are thermally adhered without using any adhesive, it is necessary to heat the liquid crystal layers that have been made into films to temperatures equal to or higher than their glass transition temperatures (Tg) to soften them. Industrialization of this method is therefore difficult from the viewpoints of the constitution of apparatus for thermal adhesion and handling. In addition, the liquid crystalline molecules in a liquid crystal layer and those in the neighboring liquid crystal layer are randomly intermingled when the liquid crystal layers are heated to high temperatures, causing deterioration of the optical properties.
Furthermore, whether an adhesive is used or not, it is necessary to employ an alignment layer and a substrate in order to align liquid crystalline molecules in the state of planar orientation. The resulting circularly-polarized-light-extracting optical element therefore has a thickness increased by the thickness of the alignment layer and that of the substrate. If an aligned film such as an oriented PET (polyethylene terephthalate) film is used as a substrate, it is possible to omit an alignment layer because the aligned film itself also serves as an alignment layer. Even in this case, the resulting circularly-polarized-light-extracting optical element has a thickness increased by the thickness of the aligned film. It seems effective that the alignment layer and the substrate are separated after the liquid crystal has been solidified. In this case, however, the liquid crystal layers are often damaged upon separation of the alignment layer and the substrate, decreasing the mass-productivity. Moreover, in the case where three or more liquid crystal layers are laminated, any of the above-described methods becomes considerably complicated; and, in addition, the substrates and the alignment layers in a number equal to the number of the liquid crystal layers are wasted.
In order to overcome these problems, there has been proposed such a method that a circularly-polarized-light-extracting layer is formed by coating a cholesteric liquid crystalline polymer layer with another cholesteric liquid crystalline polymer, as disclosed in Japanese Laid-Open Patent Publication No. 44816/1999.
This method is, however, disadvantageous in that it is difficult to always make the helical axes of cholesteric liquid crystalline molecules in the liquid crystal layers constant. Moreover, since a cholesteric liquid crystalline polymer is simply applied in the above method, the liquid crystalline molecules existing in the vicinity of the interface of two neighboring cholesteric liquid crystalline polymer layers are not oriented in one direction, and discontinuity is thus created in director at the interface of the two cholesteric liquid crystalline polymer layers. For this reason, the resulting circularly-polarized-light-extracting optical element shows lowered optical activity.